The field of the invention is nuclear magnetic resonance (NMR) imaging methods and systems. More particularly, the invention relates to an RF synthesizer and transmitter for producing RF excitation pulses having a precise frequency and phase, and a receiver for accurately receiving, demodulating and digitizing the resulting NMR signals.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomenon are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.t is tipped, and hence, the magnitude of the net transverse magnetic moment M.sub.t depends primarily on the length of time and magnitude of the applied excitation field B.sub.1 and its frequency.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited nuclei induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.t. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.-t/T2*
The decay constant 1/T.sub.2.sup.* depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the signal decays, at least in part due to a dephasing of the aligned precession of the spins after removal of the excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest. If the net magnetic moment M is not given sufficient time to relax to its equilibrium value, the amplitude A of the NMR signal produced in a subsequent pulse sequence will be reduced.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of RF excitation and a period of signal emission and acquisition. These measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more RF excitation pulses (B.sub.1) of varying magnitude, frequency content, phase and duration. Such RF excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which can produce transverse magnetization M.sub.t over a range of resonant frequencies. The prior art is replete with RF excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
After an excitation pulse, the NMR imaging system receives the radio frequency signals emitted by the excited nuclei and uses these signals to construct an image of the patient. The received signals containing "image information" lie in a band of frequencies centered at the Larmor frequency. Before the image information can be extracted to construct an image of the patient, this band of frequencies must be demodulated by shifting it to lower frequencies. Conventional signal conversion is employed to shift the band of frequencies by mixing the image information signal with a reference signal. Unless properly filtered out, noise in a band of frequencies that is symmetrical about the reference signal frequency with the image information band will become superimposed onto the image information in the resultant signal produced by the heterodyning.
This problem can be avoided if quadrature receivers are used to bring the image information to baseband. The use of in-phase (I) and quadrature (Q) demodulation allows frequencies on either side of a reference frequency to be distinguished, if the phase and amplitude adjustment of the I and Q signal channels is exact. Unfortunately such precise adjustment is difficult and misadjustment can cause some of the energy on one side of the reference frequency to be misassigned to the other side (poor "image" rejection). In addition, low frequency (e.g. 1/F) noise may be introduced into the image information signal and degrade its quality.
Heretofore, the signal processing that demodulated the image information and produced the two quadrature signals was performed in the analog domain. From a noise immunity standpoint, it is advantageous to convert the image information signal into the digital domain as early in the processing as possible. With the advent of high performance digital circuits and programmable signal processors, it is becoming possible to rapidly perform complex signal processing digitally.